Cargo compartments of military and commercial aircraft carry freight within industry-standard containers known as Unit Load Devices (ULD). The sizes and weights of ULDs vary considerably depending upon the goods transported and the characteristics of the cargo compartment. Load handling personnel must be able to rapidly load, place and unload ULDs within the cargo bay in order to minimize down time and delays.
Ball panels are among the conveyance equipment used as the primary load bearing surfaces to move ULDs in, around and out of aircraft cargo compartments where omni directional conveyance is required, such as in the doorway. Several ball panels (sometimes referred to as ball mats) can be arranged to define the deck surface of a cargo hold.
A ball panel consists of ball transfer units (BTU) and a ball panel housing. Several BTUs are inserted into openings in the top of a ball panel housing. An example of a conventional BTU is shown in U.S. Pat. No. 3,739,894, Ball Transfer Unit. A BTU comprises a case containing a ball that protrudes upwardly from the housing to contact the bottom surface of a ULD. The ball is supported resiliently within the case on a ball race or other low friction device. A standard one-inch diameter ball of the BTU is situated such that the top tangent point of the ball, which in combination with other BTUs establishes a conveyor plane, typically is 0.25 inches (6.35 mm) above the top skin of the panel. When ULDs have a planar bottom surface, this design provides 0.25 inches (6.35 mm) of clearance between a top skin of the ball panel and the ULD bottom surface.
Over time, and after exposure to arduous handling conditions, ULDs can wear or become damaged. The bottom panel of some ULD designs can sag due to use or damage, and become uneven. When the unevenness develops such that the distance between the lowest and highest point in the bottom panel of the ULD exceeds 0.25 inches (6.35 mm), a portion of the ULD will come in contact with the top skin of the ball panel, although another portion of the ULD bottom panel remains supported on the ball of the BTU. Consequently, the ULD will drag, which could lead to further damage to the ULD, as well as inhibit the ease of moving the ULD around a cargo bay. Moreover, the sagging ULD could damage the top skin of the ball panel and the BTU itself. Standard BTUs are in existing inventories of many maintenance departments. While BTUs can be replaced relatively quickly, the replacement or repair of a ball panel can be timely and expensive.
Thus there is a need for an improved ball panel design that addresses the problems caused by sagging and damaged ULDs, and that avoids costly repairs to the ball panels and their components.